Every year, plant pathogens cause billions of dollars worth of damage across the globe. Amongst the most destructive classes of plant pathogens are bacteria that grow within the xylem vessels of the host plants. As opposed to fungal pathogens, effective bactericides are not available for protecting plants against bacterial pathogens.
Most of the available compounds are antibiotics (like streptomycin and tetracycline) that are either not effective under field conditions or also affect the host plant. Moreover there is the concern that the use of these antibiotics will result in the spread of antibiotic resistance determinants amongst pathogens that cause disease in humans and other animals. For e.g., tetracycline is one of the drugs of choice for use against Yersinia pestis (the plague bacterium). The use of tetracycline against plant pathogens might result in exposure of Yersinia pestis to this antibiotic as this bacterium is harbored by rats that populate agricultural fields. There is thus a need for the identification of a new set of targets to develop novel bactericides against plant pathogenic bacteria.
Xanthomonas oryzae pathovar oryzae (Xoo) is a bacterial pathogen that causes bacterial leaf blight (BLB), a very serious disease of rice. Neither effective bactericides nor resistant rice cultivars are available to reduce yield losses due to this pathogen.
Xanthomonas oryzae pv. Oryzae causes bacterial leaf blight, a serious disease of rice. Worldwide at least 350 different plant diseases are known to be caused by various xanthomonads (17). A characteristic feature of the genus Xanthomonas is the production of yellow, membrane-bound pigments called xanthomonadins (28). The xanthomonadins are initially thought to be carotenoids, but later they are characterized as a unique group of halogenated aryl polyene pigments (2, 3).
The functional role of xanthomonadins is poorly understood. The vast majority of pigment-deficient mutants that have been isolated from several xanthomonads are prototrophs (20, 29) and virulence proficient upon wound inoculation (9, 20, and 29). Pigment-deficient mutants of Xanthomonas juglandis and X. oryzae pv. Oryzae have been reported to be more sensitive to photobiological damage than the wild-type strains are (13, 22), suggesting that the pigment may provide protection against photo-damage.
An 18.6-kb region containing seven transcriptional units required for xanthomonadin biosynthesis has been isolated from Xanthomonas campestris pv. Campestris (20, 21). One of the transcriptional units, pigB, encodes a diffusible factor that is involved in both pigment and extracellular polysaccharide production (21). The pigB mutants have also been shown to be impaired for epipyhytic survival and host infection (22).